1. Technical Field
The invention relates to a method of processing a source image (IS) comprising a step for decomposing this image (IS) into a series of “component” images (IC1, IC2, . . . , ICk, . . . , ICn) which are different, said decomposition being adapted so that the successive viewing of the images of this series at a frequency greater than the color fusion frequency for the human eye produces a fused image (IF) which is identical or quasi-identical to said source image.
2. Description of the Related Art
Such an image processing method is described in the document WO05/027529—THOMSON.
Generally, the difference between the “component” images relates to a plurality of pixels that is large enough to be more than perceptible to the eye. Thus, in cases of illegal photographing, for example using a camcorder that is not synchronized with the duly processed source images, a difference appears at the level of this plurality of pixels when illegally stored images are viewed, which degrades and considerably scrambles the viewing.
A match between the source image and the fused image means that any differences between these two images are not perceptible to the eye, so as not to degrade the direct, and therefore legal, viewing of the images.
Generally, to display an image using an image display device comprising a display screen provided with a two-dimensional matrix of elementary polychrome displays, and means for controlling each elementary display according to a video data triplet, there is associated with each pixel of this image, on the one hand, a display of the screen, on the other hand a video data triplet (DR, DG, DB) and each display of the screen with which a pixel of the image is associated is controlled using the video data triplet (DR, DG, DB) associated with this pixel. By convention, each video data triplet associated with a pixel of the image to be displayed forms, in a color space linked to the display device, the coordinates of what is called the color vector of this pixel. By definition, the set of the possible values of the video data or color vector triplets describes, in this color space associated with the device, a three-dimensional color gamut.
In the case of the display of television sequences, the video data is generally standardized, for example in the PAL system or in the NTSC system, which are both so-called luminance-chrominance systems. To display an image, a PAL image display device therefore receives, in the form of electrical signals, the video data triplets, generally denoted (Y, U, V), which correspond to the set of the color vectors and therefore of the pixels of this image, in the PAL color space linked to this device. Similarly, to display an image, an NTSC image display device therefore receives, in the form of electrical signals, the video data triplets, generally denoted (Y, I, Q), which correspond to the set of the color vectors and therefore of the pixels of this image, in the NTSC color space linked to this device. By extension, the terms YUV and YIQ spaces are used, in which Y designates the luminance, and in which U and V, or I and Q, designate the chrominance.
The video data is generally “gammatized” (set to the power of a “gamma” factor) to take account of the voltage response of cathode ray tube displays.
When it is applied to the display of an image sequence using an image display device, the image processing method described in the abovementioned document WO05/027529 results in an image display method.
As indicated in this document, such a display method then makes it possible:                on the one hand, to display any sequence of processed source images so that the observer perceives the source images of this sequence as if none of it were processed;        on the other hand, to scramble the photographing of the display of this sequence of processed images, that might be attempted by an ill-intentioned person, notably using a camcorder that is not synchronized with the sequence of source images.        
In this document, the decomposition of the source images is performed in a color space denoted YUV, that is a PAL color space; as indicated hereinabove, this space depends on the image display device used to display the images of the sequence. To decompose a pixel of the source image I1, to which corresponds a color vector of coordinates (Y1, U1, V1) in this color space, this “source” vector is decomposed into two “component” vectors respectively of coordinates (Y3, U3, V3), (Y4, U4, V4), in which Y1=Y3=Y4; again according to this document, the values of U3, V3, U4, V4 are determined so that the ends C3 and C4 of these two “component” vectors:                have a barycenter which corresponds to the end C1 of the color source vector, that is, are symmetrical relative to this end C1 in the same PAL color space linked to the display device;        are located in the color gamut of this device.        
Thus, the essential criteria for the decomposition of a color vector of a pixel of the source image into two color vectors, each associated with a pixel of a component image are that, in the color space YUV, for example, the points C1, C3 and C4 are aligned and that the Euclidian distance C1C3 is equal to the Euclidian distance C1C4.
It has been observed that the decomposition of source images as described in the document WO05/027529 did not always result in an effective scrambling of the images after illegal recording, for example using a camcorder. One aim of the invention is to enhance the scrambling of the processed images.